Both side brake lights aren't working. Light bulbs are good and fuses are good. What could it be?

i have a 1995 gmc truck backup lights not working

Have you checked the brake switch under the drivers side of the dash? There are fairly simple to check. One wire should have power all the time. The other wire should have power anytime the brake pedal is depressed. If there is no power out and power in the switch is bad. If there is no power in than there is an issue with the wiring. Let me know if you have any questions and I'll do my best to answer them. Thank you for using FixYa.com!

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Check System Fuse - A fuse is used to protect the brake light circuit from amperage overload. If the fuse has failed it will not allow the electrical current to continue to the brake lights. To check the fuse, first locate the brake light system fuse in the fuse panel which is either under the dash or under the hood in the power distribution center. Connect the test light to a ground like an engine or dash bracket and turn the ignition key to the "on" position. Using the test light probe both sides of the fuse. If the test light illuminates on both sides the fuse is okay continue onto the next step. If one side of the fuse illuminates the fuse has failed and needs to be replaced. If the fuse fails as soon as it is replaced or when you apply the brake pedal the brake light electrical circuit is shorted to ground. Rarely a system short can occur, a car repair manual is needed to find the wiring schematic for the brake light circuit. Once the brake light wiring has been located inspect and repair as needed.

Check Brake Light Bulbs - All cars have three brake light bulbs that create the brake light illumination affect. If all of these brake light bulbs fail no brake light operation will occur. I know what you're thinking, you might say all three brake lights at once? But in reality one brake light went out at a time and you did not notice. People only tend to notice something when they almost run into the back of your car to help persuade them to inform you the brake lights aren't working. Remove any of the brake light bulbs to confirm the bulb is okay or burned. After the bulb has been removed inspect the filament and replace if failed. If the brake light bulbs are okay continue to the next step.

Checking the Brake Light Switch - A control switch is used to connect the brake light electrical circuit. This switch is located near the brake pedal lever. Basic switches have just two wires, power in and power out to the turn signal switch. Use a test light that is grounded and with the key in the "on" position test for power at one side (wire) of the switch, then press the brake pedal while testing the opposite side (wire). It should illuminate the test light, if electrical power is connected through the switch go to the next step. If no power is detected through the switch the brake light switch has failed and replacement is required. If your car is equipped with more than two wires integrated into the brake light switch a car repair manual is needed to locate the proper brake light circuit wiring.

THAT RIGHT IF BRAKE SWITCH BAD NO BRAKE LIGHT WILL WORK.SPRAY ELECTRICAL CLEANER IN BULB SOCKET BULB SOCKET COULD HAVE OXIDATION AND CORROSSION CAUSING POOR CONTACT.

You do not have a separate fuse for each side, if the left side works, replace the bulb in the right side that isn't working.

Have you checked the switch on the brake pedal?
Need to find out if computer is part of brake light circuit if everything else is working.

you don`t say if its both or just one brake light out.you say the light itself is on but not if you mean tail light or what.the tail lights use a different filament in the bulb than the signal light which is the same filament that the brake light uses.if the signal light work on the rear then the bulbs are good,if just one side works then the bulb is no good on the side that doesn`t work.if the signal lights work but neither brake light works then you need to check the fuses in the fuse panel under the dash on the drivers side.if the fuses are good then it will be either a bad wire at the brake switch or a bad brake light switch,this is located on the brake pedal lever under the dash.

If the two rear brake light are working on each side, then fuse and relay are good, check the bulb.

CHECK FOR POWER AND GROUND AT THE CIRCUIT BOARD FOR THE AFFECTED BRAKE LIGHT. IF THAT CHECK OUT GOOD THEN CHECK THE BULB CONNECTION FOR BURNED CONNECTORS, THIS IS A COMMON PROBLEM. LOOPING A TEST LIGHT FROM THE POWER TO THE GROUND CONNECTION AND HOLDING THE BRAKE PEDAL DOWN IT SHOULD LIGHT. IF IT DON'T THE CIRCUIT BOARD IS BAD.

check to make sure that you used the exact bulb part number that came out. each bulb has a different resistance.